Dry etching system with inert particles coated with adsorbed acid

ABSTRACT

THIS DISCLOSURE REFERS TO OR DESCRIBES CERTAIN PRIOR ART SYSTEMS FOR ETCHING THE SURFACE OF METALS AND PLASTICS TO FORM IMPRESSIONS ON SUCH SURFACE OR TO MAKE SUCH SURFACES RECEPTIVE TO COATINGS. ALSO DISCLOSED HEREIN IS A NEW ETCHING SYSTEM WHICH IS PARTICULARLY EFFECTIVE FOR THE AFOREMENTIONED, AND OTHER, PURPOSES.

3,595,718 DRY ETCHTNG SYSTEM WITH INERT PARTICLES COATED WllTH ADSORBIED ACID David H. Fishrnan, West Orange, and Frank M. Berardinelli, South Orange, NJ., assignors to Celanese Corporation, New York, NY. No Drawing. Filed Jan. 22, 1968, Ser. No. 699,369

lint. Cl. (123E 1/00; Edie 1/00; 1324c 1/08 US. Cl. l56--2 5 Claims ABSTRACT OF THE DISCLOSURE This disclosure refers to or describes certain prior art systems for etching the surface of metals and plastics to form impressions on such surface or to make such surfaces receptive to coatings. Also disclosed herein is a new etching system which is particularly effective for the aforementioned, and other, purposes.

DESCRIPTION OF PRIOR SYSTEMS AND PROCESSES The patent literature describes a large variety of etching systems for preparing metal printing plates, and for etching metal and plastic surfaces for various purposes.

Elaborate systems requiring careful control have been devised to eliminate the problem of undercutting of the resist in the praparation of metal printing plates. For example, U.S. Pat. 3,337,462 describes and claims the use of a phosphate ester filming agent in a liquid acidic bath to prevent lateral undercutting of the resist. Acidic pastes and other liquid baths have been suggested for preparing metal and plastic printing plates. These systems usually require the use of a surfactant or other film forming material to eliminate the problem of undercutting.

As discussed hereinafter, the instant etching system provides a simple, economical method of preparing printing plates without the use of special additives to avoid undercutting.

Other types of etching systems have been used to make the surface of thermoplastics receptive to coatings. Many of these systems are generally effective but suffer from one or more disadvantages.

Recently, a process was disclosed in Us. Pat. 3,235,- 426. According to the allegations made in the specification of that patent, the new process results in a stronger bond between the paint and the resin, particularly with regard to the treatment of acetal resins and polyamide resins. In accordance with the disclosure of that patent, the new process comprises immersing a thermoplastic resin article in a solid-in-liquid dispersion which is an acidic etching medium at a temperature of from 30 to 120 C., preferably 80 to 100 C. for a brief period of time, usually 5 to 60 seconds, and thereafter removing the article from the etching medium and baking or drying it in a gaseous atmosphere for another brief period of time. Following this treatment, the residues of the etching medium are removed by washing with a suitable solvent. The baking step should be employed in order to obtain good results; or in other WOId after the article is removed from the etching medium, it should be maintained in a gaseous atmosphere at a temperature of from 50 to 200 C. for a brief period of time. Even with the baking step, the liquid bath often leaves undesirable drain lines on the surface of certain plastics.

More recently, it was discovered that the baking step could be eliminated provided the thermoplastics were etched at temperature of from about 145 to 185 C., the immersion time controlled, and a Water soluble solvent employed. This process is described in copending 3,505,?l8 Patented July 27, 197i application Ser. No. 653,042 entitled Method of Rendering Thermoplastics Receptive to Coatings, filed on July 13, 1967 in the name of Fishman et a1. Now, an even simpler, more eifective process has been discovered for preparing the surface of the thermoplastics and thermosetting materials for coating.

SUMMARY OF INVENTION The instant invention comprises the utilization of an inert, high surface area carrier impregnated with a suitable etching agent. The carrier is preferably a finely divided particulate substance, and, in accordance with this invention, is impregnated with a suitable etching agent, e.g., sulfuric acid, in an amount ranging from a small, catalytic amount to an amount of less than that amount required to agglomerate the impregnated particles; or, in other words, the impregnated particles are dry to the touch. It has been surprisingly discovered that such dry, impregnated particles are effective etching agents for a variety of purposes and oifer many advantages over the old systems.

The instant system is more versatile. It can be used as a packed bed, in a tumbling apparatus, or as a fluidized bed. The system does not require the use of toxic reagents such as tetrachloroethylene, dioxane and the like, which reagents are commonly used in liquid baths. Economic advantages are obtained because of the ease of storage and handling the dry material and because only the acid is consumed. The system can be used for preparing the surface of plastics to receive a coating without the above mentioned formation of undesirable drain lines. This is true even though a baking step is not utilized.

The most important aspect of the system is its ability to be used in a fluidized bed. Since the particles are dry to the touch, one does not experience agglomeration and therefore can utilize a fluidized system Which has many advantages. In general, a faster etch is obtained with the fluidized system. More importantly, the use of applicants fluidized system for the preparation of plastic printing plates does not cause undercutting of the resist, and therefore, one does not need an additive to provide a protective film.

DETAILED DESCRIPTION OF INVENTION The etching system comprises a high surface area carrier and a suitable etching agent. The carrier is a finely divided solid material that is inert to the etching agent and capable of adsorbing the etching agent in effective quantities. The carrier should be sufiiciently strong to avoid being broken into particles so small that they are easily agglomerated. In general, the maximum dimension of the impregnated individual particles should be in the range of from 5000 to 0.005 microns, preferably from 800 to 0.12 microns preferably spherical rather than box shaped. If the particles are substantially larger, the surface area thereof will be insufficient to give effective results.

Many materials are suitable as carriers including diatomaceous earths (kieselguhrs), such as Dicalite, manufactured by Great Lakes Carbon Corp, Super Floss and Celite manufactured by lohns-Manville Corp., minerals of a high silica content such as quartz, Cab-O-Sil, manufactured by Cabot Corporation, and Hi-Sil manufactured by Columbia Southern Chemical Co., and various clays such as attapulgite, kaolinite, and bentonite. As mentioned, this is intended to be only a representative listing which can be supplemented by the many other materials known to those skilled in the art.

Many etching agents can be used for the practice of the instant invention. The agent must be capable of etching the surface of the metal or plastic at the temperature selected for the bed, which tempreature may be in the range of from ambient to a time-temperature combination that will not cause distortion of the piece. A suitable time-temperature combination range for etching would be a temperature from 10 C. to 200 C. for a time of from 0.05 to 15 minutes. As used herein, suitable etching agent means an agent that is capable of being adsorbed in effective amounts by the selected carrier without causing substantially agglomeration of the particles. Or in other words, the impregnated particles are dry to touch. It must be understood that the impregnation process will cause initial agglomeration. But once dried, the particles can be broken into finer particles and will not thereafter agglomerate.

A variety of known acidic etching agents are suitable including organic acids such as toluenesulfonic acid, camphorsulfonic acid, naphthalene sulfonic acid and inorganic acids such as nitric and sulfuric acid, chromic acid and the like. The choice of etching agent will depend primarily on the surface to be treated. The preferred agents are described hereinafter in the discussion relating to specific applications.

For the treatment of certain substrates a variety of caustic agents are suitable, e.g. tributylarnine.

The impregnated particle must be dry to the touch in order to avoid agglomeration of the particles. Thus the relative amounts ofcarrier and etching agent must be carefully selected. In general, the carriers described herein are capable of adsorbing large amounts of agent. Thus the composition may contain from about 0.01 to 80% by Weight acid and from about 99.99 to 20% by weight solid particles. Certain carriers, however, are less adsorbant for certain agents. Thus other compositions may comprise from about 5 to 50% by weight etching agent and 95 to 50% by weight solids. The adsorbance of the selected solid can be easily determined and the relative amounts calculated. In the examples given hereinafter, typical ef fective relative proportions are shown.

The etching agent can be adsorbed onto the solid particles in a variety of ways. For example an acid, such as solid toluenesulfonic acid, can be first dissolved in a suitable solvent such as methanol. The solid particles, e.g., Cab-O-Sil, are then added to the solution for a time suflicient to impregnate said particles. Impregnation is aided by rapid stirring. The solvent is then removed from the system in a rotary evaporator or by any other suitable operation. The particles are then dried and passed through a sieve, e.g. 30 mesh, to get particles of the desired size. If an excess of agent is used, the particles will agglomerate. They can be easily dried and broken up into smaller particles.

The present invention provides an effective, economical procedure for etching the surfaces of thermoplastics and metals. The method comprises contacting the surface of the material with a bed of finely divided solid particles having a suitable etching agent adsorbed thereon. The treatment can be performed in a packed bed by merely immersing or burying the material in a bed of the impregnated particles, by tumbling the piece in a bed of the impregnated particles, or by treating the piece with a fluidized bed of impregnated particles.

In the packed bed system, the impregnated particles are packed tightly against the surface of the piece. In the fluidized bed system the particles are agitated in a tank by the use of a suitable gas, for example, air or an inert gas such as nitrogen. The amount of particles used in the fluidized bed will depend, of course, on the size of the system. The amount of particles and the rate of flow of the fluid must be chosen to assure good contact between the impregnated particles and the materials being treated.

The materials may be treated by merely tumbling the same in the presence of the impregnated particles. In this system, the pieces to be treated are charged into an apparatus capable of rotary motion. The impregnated particles are added to the system a d the apparatus turns the pieces, thereby bringing the impregnated particles in conact with the pieces.

A wide variey of thermoplastics may be treated in accordance with the general process of this invention, for example, polyesters, polyamides and polyacetals. The process is particularly well suited to the treatment of acetal resins. The acetal resins contemplated for use in this invention are the addition homopolymers, copolymers, terpolymers and the interpolymers of aldehydes such as formaldehyde and cyclic oligomers such as trioxane. Two of the acetal resins that are available commercially are Celcon acetal copolymer (Celanese Corporation) and Delrin acetal resin (E. I. du Pont de Nemours & Co.). Celcon is described and claimed in US. Pat. 3,027,352. Metals such as aluminum, iron, steel and magnesium can also be etched with the instant composition. Particularly rapid results are obtained with the use of a fluidized bed.

As mentioned above, the composition of this invention, can be used to etch metal and plastics for a variety of purposes including improving adhesion of coatings, making printing plates, printed circuits and the like. Of particular interest are the etching to improve adhesion and the preparation of printing plates. These two applications are hereafter treated in detail.

IMPROVING ADHESION OF PLASTICS AND METALS The metal or plastic surface can be effectively rendered receptive to coatings by any of the methods suggested herein, packed bed, tumbling, or fluidized bed.

Currently, steel and other metals are pickled i.e. cleansed and oxides removed from the surface by contact with a strong acid. The use of the instant system is effective for this purpose. An acid, for example, nitric acid, is adsorbed on a high surface area material such as Cab-O- Sil and this system is used either at ambient or elevated temperatures in a packed bed, tumbling apparatus, fluidized bed or even spray-blasted on the surface of the metal to be treated. The instant system provides ease of handling since it is a dry system. The instant system can also be used in metal plating processes. For example, zinc powder is currently plated on steel by tumbling with abrasive cones and a liquid acid. By substituting the instant system, one would be able to utilize a dry process. In addition the instant system can be used to replace the use of a liquid acid to strip surface paint from metal surfaces. One of the primary uses envisaged for treatment of metal surfaces with the instant system is to prepare such surfaces to receive a coating such as paint or the like. The following example demonstrates the eflicacy of the instant system in promoting the adhesion of paint to aluminum, iron and stainless steel.

EXAMPLE I The system comprising 20% sulfuric acid and Cab-O-Sil was prepared by dissolving a weighed portion of the acid in methanol, adding the calculated weight of Cab-O-Sil and stripping the methanol in a rotary evaporator. The semi-dry impregnated material was dried in a vacuum oven at 60 C. overnight. After drying it was broken into fine particles. Small pieces mils by A2 inch by 3 inches) of aluminum, stainless steel and iron were buried to a depth of about 2 inches for 30 minutes. They were then removed, washed well with water, dried and both the treated and untreated surfaces of each plate were painted. The treated surfaces had excellent adhesion in the case of aluminum and very good adhesion in the cases of iron and stainless steel. Adhesion on the treated areas was better than that obtained on the untreated areas.

The method used for etching plastics will depend upon the end use. Acetal resins treated in a packed bed exhibit excellent initial adhesion and excellent adhesion after a 10 day water immersion test. Acetal resins treated in a fluidized bed exhibit excellent initial adhesion but the adhesion. measured after the 10 day immersion test was not FLUIDIZED BED-ETCHING RATE Etch rate, mils per minute at- H2SO4 100 C. 120 0. 140 C. 150 C.

The following examples illustrate the efficacy of the instant invention. Several samples of impregnated Cab-O- Sil were prepared having varied concentrations of toluenesulfonic acid and sulfuric acid. The samples were prepared by dissolving the acid in methanol and adding the calculated weight of Cab-O-Sil with every rapid stirring for approximately 10 minutes. The solvent was then removed in a rotary evaporator. After keeping the semi-dry acid-on-Cab-O-Sil in a vacuum oven at 60 C. overnight, the samples were broken up into fine particles and used for etching as described in the Examples I, IV, V, VIII.

EXAMPLE II Several panels (1" x 3"-60 mil thickness) of Celcon acetal copolymer and Delrin acetal resin were buried in a bed of Cab-O-Sil having toluenesulfonic acid adsorbed thereon. The bed comprised 20% p-toluenesulfonic acid and 80% Cab-O-Sil. The panels were buried for 8, 10 and 12 minute intervals at a temperature of 90 C. After being removed, the panels were quenched and washed well in water. They were thereafter air dried, degreased, painted and adhesion tested both initially and after ten day water immersion at 90 F. Two tests were performed: The Scotch Tape Test comprised scoring the surface of the panel with a razor blade, cutting through the parallel line at about apart to get a pattern. Scotch tape was applied over the scored area with manual pressure and quickly peeled off. If no paint was removed, the surface was considered to have passed the test. In the Nickel Test a nickel was manually run along the surface to be tested in an attempt to dislodge the paint. If no plastic surface was exposed, the sample was considered to have passed the test. As mentioned above, these tests were performed immediately after the paint dried and again after the painted samples had been immersed for ten days in Water at 90 F. All of the panels, Celcon and Delrin passed both the initial and the ten day test. Untreated Celcon and Delrin plaques were painted and tested as described above. The untreated plaques had extremely poor paint adhesion both initially and after a ten day water exposure.

As mentioned hereinbefore the US. Pat. No. 3,235,426 suggests reacting thermoplastics with an acidic etching medium comprising a solids in liquid dispersion. The following example shows that it is necessary to bake or dry the thermoplastic object following the etching procedure taught and claimed by that patent.

EXAMPLE III A dispersion of solids-in-all liquid was prepared having the following composition in parts by weight.

Tetrachlorethylene 96.2

Samples of Celcon acetal copolymer and Delrin acetal resin were immersed in the bath at 113 C. for 5, 30 and 60 second intervals. Samples were then either water quenched and washed well or over-baked at 120 C. for 60 seconds and then washed. They were painted and tested for adhesion both immediately and after 10 day water immersion at 90 F. Excellent results were obtained both for immediate adhesion and for the ten day test in those instances where the overbake or drying step was employed. However, those samples that were immediately water quenched and washed yielded poor results.

EXAMPLE IV Example II demonstrates the efficacy of a packed bed system. To determine wether or not a fluidized bed system would work, Cab-O-Sil having 20% adsorbed toluenesulfonic acid based upon total weight of and Cab-O-Sil and toluenesulfonic acid was charged into an Armstrong Vibro-Fluidizer. Compressed air was then fed into the fluidizer at a pressure of 45 p.s.i..g. through 50 feet of a quarter-inch inside diameter copper tubing coil maintained in a silicone oil bath heated to 180 C. By a careful temperature control, a thermocouple reading of 90 C. was achieved in the fluidized solidin-gas bed. Panels (60 mil thickness-3 x 3") of Celcon and Delrin were immersed for 1 /2, 2, 2 /2, 3, 8 and 12 minute intervals. These were quenched and tested as discussed in Example I. All samples demonstrated excellent initial adhesion. The samples that were immersed in Water for 10 days demonstrated adhesion that was inferior to that obtained from the packed bed treatment.

EXAMPLE V The test described in Example IV were repeated with the use of compressed nitrogen rather than compressed air in the fluidizer. Adhesion tests were performed, and as expected, the initial tests were again satisfactory. The samples that were immersed in water for 10 days demonstrated adhesion that was superior to the results obtained in Example IV. It is therefore clear that it is preferred to use an inert atmosphere in the fluidized system.

EXAMPLE VI The tests described in Example V were performed using sulfuric acid rather than toluenesulfonic acid. The use of sulfuric acid resulted in a demonstration of markedly improved adhesion, especially after the 10 day water immersion.

EXAMPLE VII A variety of levels of toluenesulfonic acid on Cab-O-Sil were prepared and evaluated for etching efficiency at temperatures of C. and C. in a packed bed. These samples include a zero percent control, 2%, 5%, 10%, 20%, 40% and 50% acid on CabO-Sil. Treatment with Cab-O-Sil (control, no acid) at C. for 30 minutes provided no visible change to the sample and imparted no improvement to adhesion characteristics. Levels of 2%, and 5% did not appear to be adequate to achieve rapid etching, but at a concentration of at least 10% toluenesulfonic acid on Cab-O-Sil, sufiicient etching could be realized in approximately eight minutes at 90 C. These samples exhibited excellent initial adhesion and excellent ten day water immersion stability.

THE PRINTING PLATE The etching system of this invention is eifective for preparing printing plates made of plastic, particularly polyacetals. The procedure comprises forming a relief or impression on molded plastic and then etching the areas not protected by the resistant material.

The particles are then dried and passed through a sieve, e.g., 30 mesh, to get particles of the desired size. If an excess of agent is used, the particles will agglomerate. They can be easily dried and broken up into smaller particles.

The resist has two requirements. It must be capable of good adhesion with the substrate and it must be stable to the etching environment. For a deep etch, for example, 30 mils or more, tape and ink are preferred. One suitable tape is No. 471 Scotch Brand tape. Tech-Pen Ink sold by Mark-Tex Corp. is a suitable resist which can be used for deep etches. Both of these types of materials withstand the etching environment and adhere to acetals and other plastics under the strenuous conditions required in the etching system. There are a variety of other materials that are available commercially; for example the following commercial photo resists Withstand the etching environment and adhere well for etches up to about 6 mils or more: Kodak Photo Resist KPR3 and Waycoat 10 and Waycoat 20 sold by Phillips Hunt Chemical Co. If a photo resist is employed in the molded acetal is coated with a light sensitive coating or enamel. The coated surface is then exposed to light through a negative having an image thereon in order to project a corresponding image in the coating. It is preferred that the plastic substrate be opaque in this system. The acetal resin may be made opaque in various ways, e.g. by incorporating a minor amount of a filler into the resin. The exposed, coated surface is then developed in order to form an acid resistant coating in the form of the image produced by the exposure. This acid resistant coating can be further hardened by heating.

The unprotected surfaces are then etched rapidly and conveniently by contacting the surface with a fluidized bed of acid adsorbed on a high surface area carrier. A preferred etching system consists of 50% sulfuric acid on Cab-O-Sil (based on total weight of system). The use of this system at 100 C. to 150 C. utilizing either air or an inert fluidizing medium, will give a very shallow etch in 10 seconds and a very deep etch if treatment is continued for 15 minutes. For most printing plate operations, a suitable etch may be obtained by treating the acetal surface for from 1 to 5 minutes. The etched surfaces are then Water washed, dried and used as printing plates. A packed bed or tumbling procedure may also be employed. These systems do not cause undercutting, but the rate of etch is much less than the fluidized system.

The following examples demonstrate the efficacy of the instant system for the preparation of plastic printing plates.

EXAMPLE VIII An etching system comprising 50% sulfuric acid and 50% Cab-O-Sil (prepared in the same manner as that used in Example I) was used to etch a Celcon plaque in a fluidized bed utilizing compressed air (45 p.s.i.g.) at 140 C. Before etching, a pattern was formed on each piece of Celcon. On some samples Scotch Brand tape was used as the resist to form the pattern and in other samples ink from a Tech-Pen manufactured by Mark-Tex Corp. was used to form the pattern. In addition, since under these conditions, etching is on all sides of the samples, the back side of the sample was covered with tape so that only the unprotected portion surrounding the resist was etched. Under these conditions, the unprotected area was etched away to varying degrees so that a raised image was formed. This raised image can be coated and used for printing. The following table lists depth of each for the given time of etch for the taped samples.

Time of etch (min): Depth of etch (mils2) The depth of etch was not measured for the samples containing the ink resist. However, those samples were treated for the same intervals. In all cases, a rapid etch was obtained. There was no undercutting and the resists adhered well to the surfaces of the Celcon and were stable to the etching environment.

EXAMPLE IX To determine image quality and resist stability to the instant etching system, a system comprising 50% H 50 on Cab-O-Sil was used in a fluidized bed. Samples of Celcon were treated with Waycoat 10 and Waycoat 20 photo sensitive resists acquired from Phillip A. Hunt Chemical Co. Each sample was dip coated in the photo resist. After air drying and preheating at 100 C. for 15 minutes, the samples were exposed through a cellulose base negative at 15 inches for 5 minutes using a 275 watt sunlamp. The exposed surfaces were then developed in Waycoat Photo Resist Developer for 3 minutes and dipped into Waycoat Photo Resist black dye for 15 seconds. The samples were washed well in deionized water, air dried and dried with an air heat gun for approximately 15 minutes. These samples were then etched at 100 C. with the above described system for two minutes. The resist adhered well to the Celcon sample and was stable to the etching atmosphere. In addition, the absence of undercoating was observed. The tests were repeated With Kodak photo resist and the same results were observed.

EXAMPLE X A packed bed was utilized to determine etching rate and occurrence of undercutting. Celcon plaques (3" x 3 x 0.06") were backed with acid resistant tape and the fronts were covered with a circle of acid resistant tape. The plaques were packed in a bed of impregnated particles comprising 50% concentrated sulfuric acid and 50% Cab- O-Sil for 2, 5, 10 and 15 minute intervals. At the end of 15 minutes, 10 mils had been etched away. It was observed, however, that no undercutting occurred. Thus, it is clear that both the packed bed and fluidized bed can be used to prepare printing plates because neither system caused undercutting. It is clear, however, that the fluidized bed is preferred because a much more rapid etch is obtainable.

It should be pointed out that an excellent resolution is obtained by using the impregnated particles of this invention. Whereas, the use of liquid etching agents or pastes often result in poor resolution. The description herein, re lating to the preparation of printing plates has dealt primarily with the preparation of plastic printing plates. The etching system of this invention can be used to prepare metal printing plates. A particularly desirable system is the use of Cab-O-Sil impregnated with nitric acid to etch copper, brass or magnesium substrates.

This invention has been described to a certain degree of particularity. Many variations will be apparent to those skilled in the art without departing from the scope of the invention which is defined in the following claims.

The embodiments of the invention in which an exclusive property is claimed are defined as follows:

1. A method of treating the surface of an article fabricated from a thermoplastic resin selected from the group consisting of oxymethylene homopolymers and copolymers in order to improve the receptivity of said surface to coating comprising, contacting said surface with a dry etching bath at a temperature of from 10 C. to 200 C. for a time of from 0.05 to 15 minutes; said dry etching bath comprising discrete finely divided inert particles having maximum dimensions of from 5000 to 0.005 microns, having adsorbed thereon from about 0.01 to percent by weight of an acid etching agent, based upon the combined Weight of the inert particles and the acid etching agent, wherein said dry etching bath is in the form of a fluidized bed, and the fluidizing medium is an inert gas.

2. The method of claim 1 wherein said inert particles have maximum dimensions of from 800 to 0.12 microns.

3. The method of claim 2 wherein said acid etching agent is selected from the group consisting of sulfuric acid and toluenesulfonic acid.

4. The method of claim 3 wherein said acid etching agent is present at from 5 to 50% by weight, based upon the combined weight of the inert particles and the acid etching agent.

5. A method for etching a surface which is susceptible to acidic etching selected from the group consisting of oxymethylene homopolymers and copolymers which comprises contacting said surface With a dry etching bath comprising discrete finely divided inert particles having maximum dimensions of from 5000 to 0.005 microns having adsorbed thereon from about 0.01 to 80 percent by weight of an acid etching agent, based upon the combined weight 1 References Cited UNITED STATES PATENTS 0 JACOB H. STEINBERG, Primary Examiner US. Cl. X.R. 

